The present invention relates to a device for generating acoustic pulses by implosion inside a well or a cavity, and, more particularly, to an emission device adapted to be lowered to a certain depth in a well drilled in the ground, for generating acoustic pulses which may be used for carrying out, for example, seismic prospection.
A seismic source for wells has a number of advantages as compared with a source disposed on the surface. An appreciable part of the acoustic energy produced by a surface source is in fact emitted in the form of surface waves or RAYLEIGH waves which cannot be used and the part of this energy which is emitted in the form of P or S type waves is considerably attenuated by the surface layer weathered zone.
The P or S wave energy yield of a well source emitting under the surface layer is therefore much larger.
Among well sources of a known type, there may be mentioned those using solid explosives or explosive gas mixtures. Such sources produce very powerful seismic pulses but use thereof at a great depth is difficult because the duration of their operating cycle is fairly long. It should also be noted that the frequency spectrum of these pulses comprises a considerable part of high frequencies which have no interest in the field of seismic prospection. In addition, the power of the explosions generally results in damaging the well or impairing the neighboring regions, so that subsequency repetition of seismic firing at the same positions is difficult. Explosive sources are described for example in U.S. Pats. No. 3,408,622 and 3,858,167.
Sources of the air gun type may also be mentiioned in which the seismic pulses are generated by sudden gas discharges inside the well filled with water. Such sources produce powerful acoustic waves but require the use of a compressor or compressed fluid reservoirs which, because of the smallness of the drilled hole, must be installed on the surface and connected to the source by piping which also makes use thereof difficult and appreciably increases the duration of their operating cycle. Such a source is described for example in French Pat. No. 2,311,322.